1. Field of the Invention
This invention relates to a process for the production of detergent tablets by microwave and hot air treatment.
The disadvantage of conventional detergent tablets which are normally produced by compression molding or fusion is that they do not dissolve sufficiently quickly on account of their compactness so that the active substances are released too slowly. In addition, the rate at which such tablets disintegrate is too low.
2. Discussion of Related Art
Earlier hitherto unpublished International patent application PCT/EP94/01330 now WO 94/25563, to the disclosure of which reference is expressly made, describes in detail the production of washing- and cleaning-active tablets using microwaves which have an extremely high dissolving or disintegrating rate coupled with high breaking strength. A crucial requirement for the production of tablets from powder-form or granular raw materials using microwaves is that the starting materials should be at least partly present in hydrated form, "hydrated" meaning "hydrated under certain conditions in regard to temperature, pressure or relative atmospheric humidity to which the raw material is exposed or with which the raw material is in equilibrium". The term "hydrated" is also defined in PCT/EP94/01330. In general, hydrated starting materials are those which contain bound water of crystallization or which are capable of binding externally added water at least partly as water of crystallization or even those substances which do not form defined hydrates, but which are capable of binding water, for example alkali metal hydroxides.
The expression "microwaves" in the context of the present invention is understood to cover the entire frequency range from 3 to 300,000 MHz, i.e. the frequency range which, in addition to the actual microwave range above 300 MHz, also encompasses the radio wave range from 3 to 300 MHz. This technique can be used to produce so-called macrosolids which, besides tablets, also include blocks for example. To this end, the compounds are joined together at their points of contact with one another by local microwave-induced melting/sintering. The voids present between the individual components of the compounds before exposure to microwaves provide the tablets formed with high porosity and thus contribute towards improving the dissolving properties of the tablets.
To facilitate local sintering of the various components of the compounds, at least some of the components must be capable of sintering at their surface. To this end, the components of the compounds themselves or their surfaces must contain sufficient water so that the components of the compounds melt at their points of contact when the water is heated. According to the teaching of International patent application PCT/EP94/01330, the mixture to be exposed to microwaves must be at least partly present in hydrated form.
In the context of the present invention, therefore, the term "tablets" is not confined to any particular three-dimensional form. In principle, the tablets may assume any three-dimensional form, depending on the shape which the powder-form or granular compounds are made to assume.
The chemical composition of the generally powder-form or granular compounds--and hence the tablets--can be varied over a very broad range, cf. the disclosure of PCT/EP94/01330.
It has now been found that tablets produced by the microwave treatment of powder-form or granular compounds on the one hand lack the breaking strength required for storage and transport if the microwave treatment is too short and, on the other hand, undergo core carbonization if the microwave treatment is too long. Hitherto, it has now always been possible to solve this problem because, in many cases, adequate breaking strength inevitably involved carbonization within the tablet and the avoidance of carbonization resulted in inadequate breaking strength.
Accordingly, the problem addressed by the present invention was to find a process in which the disadvantages mentioned above would not arise, i.e. which would give tablets combining a high breaking strength with the absence of any carbonization.